Semiconductor memory device

ABSTRACT

A semiconductor memory device includes a memory cell array in which memory cells are arranged, and a first wiring connected to the memory cells. A discharging circuit discharges the voltage of the first wiring according to a first current. In addition, a charging circuit charges the voltage of the first wiring according to a second current. A control circuit detects the voltage of the first wiring and controls a magnitude of the second current based on the detected voltage. A current detection unit generates a third current proportional to the second current and generates a detection result based on a magnitude of the third current. The discharging circuit is configured to control a magnitude of the first current in accordance with the detection result.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-188530, filed Aug. 29, 2012; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate to a semiconductor memory device.

BACKGROUND

A NAND-type flash memory is a well-known semiconductor memory devicethat can store data in a nonvolatile fashion with high storage capacity.A cell array of NAND-type flash memory is formed by arranging NAND cellunits in which several memory cells are connected in series. Word andbit lines are used to read and write data to specific memory cells. Wordlines connect memory cells in different cell units and bit lines eachconnect to a different cell unit.

A potential problem with NAND-type flash memory is that a parasiticcapacitance between adjacent or near-by word lines increases as devicefeature sizes decrease as a result of device miniaturization. As morememory cells and, therefore, word lines are included in smaller spacesthere are increasing problems with parasitic capacitance between wordlines, which may adversely affect device performance.

Due to parasitic capacitance, when a desired (predetermined) voltage forreading or writing data is supplied to selected word lines, a voltageovershoot is sometimes caused in the selected word lines by capacitivecoupling with adjacent word lines. If the overshoot is large, deviceresponse time is delayed until the desired voltage is actually obtained.Delayed response times can cause poor device performance. Thiscapacitive coupling phenomenon is especially distinct inthree-dimensional NAND-type flash memory arrays.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a nonvolatile semiconductor memorydevice according to a first embodiment.

FIG. 2 is an oblique view showing a layered structure of a memory cellarray according to the first embodiment.

FIG. 3 is a circuit diagram depicting a memory array.

FIG. 4 is a cross section depicting the memory array.

FIG. 5 is a partial enlargement of a portion of FIG. 4.

FIG. 6 is a top view depicting a second source side conductive layer anda second drain side conductive layer.

FIG. 7 is a circuit diagram depicting a control circuit.

FIG. 8 is a circuit diagram depicting a step-up circuit according to thefirst embodiment.

FIG. 9 is a timing chart depicting an example operation of the firstembodiment.

FIG. 10 is a circuit diagram depicting the step-up circuit according tothe first embodiment.

FIG. 11 is a circuit diagram depicting a step-up circuit according to asecond embodiment.

FIG. 12 is a circuit diagram depicting the step-up circuit according tothe second embodiment.

DETAILED DESCRIPTION

In the semiconductor memory device of the present disclosure, theperformance of the device can be improved by reducing response timedelays due to voltage overshoots. In general, the nonvolatilesemiconductor memory device of the present disclosure will be explainedby reference to example embodiments depicted in the drawings.

A semiconductor memory device includes a memory cell array in whichmemory cells are arranged, and a first wiring which is connected to thememory cells. A discharging circuit discharges the voltage of the firstwiring according to a first current. In addition, a charging circuitcharges the voltage of the first wiring according to a second current. Acontrol circuit detects the voltage of the first wiring and controls thecharging of the first wiring. A current detection unit generates a thirdcurrent proportional to the second current and generates a detectionresult in accordance with the size of the third current. The dischargingcircuit is configured to control the size of the first current inaccordance with the detection result from the current detection unit.

(First Embodiment)

First, the nonvolatile semiconductor memory device of the firstembodiment will be explained with reference to FIG. 1. FIG. 1 is acircuit diagram depicting the nonvolatile semiconductor memory deviceaccording to the first embodiment.

As shown in FIG. 1, the nonvolatile semiconductor memory device of thefirst embodiment has a memory array AR1 and a control circuit AR2installed at the periphery of memory array AR1.

The memory cell array AR1 is constituted by several memory strings MS inwhich electrically rewritable memory transistors MTr1-8 (memory cells 1through 8) are connected in series. The control circuit AR2 comprisesvarious kinds of control circuits for controlling voltages applied tothe gates of the memory transistors MTr (MTr1-8).

The control circuit AR2 implements an operation for writing data to thememory transistors MTr, an erasing operation for erasing data in thememory transistors MTr, and an operation for reading data from thememory transistors MTr. At a time of the write operation and the readoutoperation, a voltage is applied to selected memory strings MS. Thisvoltage is approximately that used in a conventional stacked flashmemory device.

The memory cell array AR1 has m columns of memory blocks MB. Each memoryblock MB has n rows and two columns of memory units MU. The memory unitMU includes a memory string MS, first source side select transistorSSTr1 connected in series to the source end of the memory string MS,second source side select transistor SSTr2, first drain side selecttransistor SDTr1 connected in series to the drain end of the memorystring MS, and second drain side select transistor SDTr2. As an example,there are two each of the drain side select transistors and source sideselect transistors in one memory string MS; however, this is only anexample, and one each of the drain side select transistor and the sourceside select transistor may be used. Here, in the example shown in FIG.1, the first column of the memory units MU is labeled (1), and thesecond column is labeled (2). In each memory block MB, two memory unitsMU arranged in the column direction share the bit line BL. In eachmemory block MB, n pieces of memory units MU parallel with the rowdirection share word lines, select gate lines, source lines, and backgate lines. Bit lines BL and source lines SL are shared by the m columnsof memory blocks MB.

The memory cell array AR1, as shown in FIG. 2, is constituted by memorytransistors MTr in a three-dimensional matrix form. In other words, thememory transistors MTr are arrayed in a matrix in the horizontaldirection and also arrayed in a layering direction (the directionperpendicular to a substrate, also referred to as the “stackingdirection”). Several memory transistors MTr1-8 parallel in the stackingdirection are connected in series, constituting memory strings MS. Thefirst and second drain side select transistors SDTr1 and SDTr2 and thefirst and second source side transistors SSTr1 and SSTr2, which are setto a conductive state when they are selected, are connected to both endsof the memory strings MS. These memory strings MS are extend in thestacking direction as a longitudinal direction. Next, the circuitconfiguration of the memory cell array AR1 will be explained withreference to FIG. 3. FIG. 3 is an equivalent circuit diagram showing thememory array AR1.

The memory cell array AR1, as shown in FIG. 3, has several bit lines BLand several memory blocks MB. The bit lines BL are arranged at aprescribed pitch in the row direction and formed extending in the columndirection as a longitudinal direction. The memory blocks MB are arrayedat a prescribed pitch in the column direction.

The memory blocks MB have several memory units MU arranged in a matrixform in the row direction and the column direction. For the memory blockMB, several memory units MU commonly connected are installed in onepiece of bit line BL. A memory unit MU has a memory string MS, a firstsource side select transistor SSTr1, a second source side selecttransistor SSTr2, a first drain side select transistor SDTr1, and asecond drain side select transistor SDTr2. These memory units MU arearranged in a matrix form in the row direction and the column direction.

The memory string MS includes memory transistors MTr1-8 connected inseries and a back gate transistor BTr. The memory transistors MTr1-4 areconnected in series in the stacking direction. The memory transistorsMTr5-8 are similarly connected in series in the stacking direction. Inthe memory transistors MTr1-8, a threshold voltage is changed byaccumulated electric charges stored in charge storage layers. Bychanging of the threshold voltage, data that are stored in the memorytransistors MTr1-8 are rewritten.

A back gate transistor BTr is connected between the memory transistorMTr4 and the memory transistor MTr5. This connection is made on thelowermost layer. Therefore, the memory transistors MTr1-MTr8 and theback gate transistor BTr are connected to form a U shape when viewed incross section along the column direction.

The drain of the first source side select transistor SSTr1 is connectedto one end of the memory string MS (the source of the memory transistorMTr8). The drain of the second source side select transistor SSTr2 isconnected to the source of the first source side select transistorSSTr2. The source of the first drain side select transistor SDTr1 isconnected to the other end of the memory string MS (the drain of thememory transistor MTr1). The source of the second drain side selecttransistor SDTr2 is connected to the drain of the first drain sideselect transistor SDTr1. These transistors SSTr1, 2 and SDTr1, 2 havedifferent threshold voltages due to the amount of electric charges thatare accumulated in each charge storage layer.

The gates of n memory transistors MTRrl arranged in one column in therow direction are commonly connected to one word line WL1 extending inthe row direction. Similarly, the gates of n memory transistors MTr2-8arranged in one column in the row direction are respectively, commonlyconnected to one respective word line WL2-8 extending in the rowdirection. In addition, the gates of 2×n back gate transistors BTrarranged in a matrix form in the row direction and the column directionare commonly connected to a back gate line BG.

The gates of n first source side select transistors SSTr1 arranged inone column in the row direction are commonly connected to one firstsource side select gate line SGS1 extending in the row direction.Similarly, the gates of n second source side select transistors SSTr2arranged in one column in the row direction are commonly connected toone second source side select gate line SGS2 extending in the rowdirection. In addition, the sources of the second source side selecttransistors SSTr2 are connected to source lines SL extending in the rowdirection.

The gates of n first drain side select transistors SDTr1 arranged in onecolumn in the row direction are commonly connected to one first drainside select gate line SGD1 extending in the row direction. The gates ofn second drain side select transistors SDTr2 arranged in one column inthe row direction are commonly connected to one second drain side selectgate line SGD2 extending in the row direction. In addition, the drainsof the second drain side select transistors SDTr2 are connected to thebit lines BL extending in the column direction.

Next, the layered structure of the nonvolatile semiconductor memorydevice of the first embodiment will be explained with reference to FIG.4 and FIG. 5. FIG. 4 is a cross section showing the memory cell arrayAR1, and FIG. 5 is a partial enlargement of a portion of FIG. 4.

The memory cell array AR1, as shown in FIG. 4, has a back gatetransistor layer 20, a memory transistor layer 30, a select transistorlayer 40, and a wiring layer 50 on a substrate 10. The back gatetransistor layer 20 functions as the back gate transistor BTr. Thememory transistor layer 30 functions as the memory transistors MTr1-8(memory string MS). The select transistor layer 40 functions as thefirst source side select transistor SSTr1, the second source side selecttransistor SSTr2, the first drain side select transistor SDTr1, and thesecond drain side select transistor SDTr2. The wiring layer 50 functionsas the source lines SL and the bit lines BL.

The back gate transistor layer 20 has a back gate conducting layer 21.The back gate conducting layer 21 functions as the back gate line BG andfunctions as the gate of the back gate transistor BTr.

The back gate conducting layer 21 is formed so that it is extends in therow direction and the column direction parallel with the substrate 10.The back gate conducting layer is divided for each memory block MB. Theback gate conducting layer 21 is composed of polysilicon (poly-Si), forexample.

The back gate conducting layer 20, as shown in FIG. 4, has a back gateholes 22. The back gate holes 22 are formed so that the back gateconducting layer 21 fills them. The back gate hole 22 is formed in anapproximate rectangular shape in which the column direction is alongitudinal direction from the top view. The back gate holes 22 areformed in a matrix form in the row direction and the column direction.

The memory transistor layer 30, as shown in FIG. 4, is formed on theupper layer of the back gate conducting layer 20. The memory transistorlayer 30 has word line conductive layers 31 a-31 d. The word lineconductive layers 31 a-31 d, respectively, function both as the wordlines WL1-8 and as the gates of the memory transistors MTr1-8.

The word line conductive layers 31 a-31 d are layered via an interlayerdielectric (not shown in the drawing). The word line conductive layers31 a-31 d are formed so that they have a prescribed pitch in the columndirection and extend in the row direction as a longitudinal direction.The word line conductive layers 31 a-31 d are composed of polysilicon(poly-Si).

The memory transistor layer 30, as shown in FIG. 4, has a memory hole32. The memory hole 32 is formed so that it penetrates through the wordline conductive layers 31 a-31 d and the interlayer dielectric (which isnot depicted in the drawing). The memory hole 32 is formed so that it ismatched with the end vicinity in the column direction of the back gatehole 22.

In addition, the back gate transistor layer 20 and the memory transistorlayer 30, as shown in FIG. 5, have a memory gate insulating layer 33 anda memory semiconductor layer 34. The memory semiconductor layer 34functions as the body of the memory transistors MTr1-MTr8 (memory stringMS).

The memory gate insulating layer 33, as shown in FIG. 5, is formed at aprescribed thickness on the side surfaces of the back gate hole 22 andthe memory hole 32. The memory gate insulating layer 33 has a blockinsulating layer 33 a, a charge storage layer 33 b, and a tunnelinsulating layer 33 c. As electric charges are accumulated in the chargestorage layer 33 b, the threshold voltage of the memory transistorsMTr1-8 is changed, rewriting data that are held in the memory transistorMTr.

The block insulating layer 33 a, as shown in FIG. 5, is formed at aprescribed thickness on the side surfaces of the back gate hole 22 andthe memory hole 32. The charge storage layer 33 b is formed at aprescribed thickness on the side surface of the block insulating layer33 a. The tunnel insulating layer 33 c is formed at a prescribedthickness on the side surface of the charge storage layer 33 b. Theblock insulating layer 33 a and the tunnel insulating layer 33 c arecomposed of a silicon dioxide (SiO₂). The charge storage layer 33 b iscomposed of a silicon nitride (SiN). Those skilled in the art willrecognize that other materials may be suitable for the insulation andstorage layers.

The memory semiconductor layer 34 is formed so that it is in contactwith the side surface of the tunnel insulating layer 33 c. The memorysemiconductor layer 34 is formed so that the back gate hole 22 and thememory hole 33 are buried in it. The memory semiconductor layer 34 isformed in a U shape in the row direction. The memory semiconductor layer34 has a pair of columnar parts 34 a extending in the directionperpendicular to the substrate 10 and a connecting part 34 b forconnecting the lower ends of a pair of columnar parts 34 a. The memorysemiconductor layer 34 is composed of polysilicon (poly-Si).

In other words, in the constitution of the back gate transistor layer20, the memory gate insulating layer 33 is formed so that it surroundsthe connecting part 34 b. The back gate conducting layer 21 is formed sothat it encloses the connecting part 34 b via the memory gate insulatinglayer 33. In addition, in the constitution of the memory transistorlayer 30, the memory gate insulating layer 33 is formed so that itencloses the columnar part 34 a. The word line conductive layers 31 a-31d are formed so that they surround the columnar part 34 a via the memorygate insulating layer 33.

The select transistor layer 40, as shown in FIG. 4, has a first sourceside conductive layer 41 a and a first drain side conductive layer 41 b.The first source side conductive layer 41 a functions as the firstsource side select gate line SGS1 and functions as the gate of the firstsource side select transistor SSTr1. The first drain side conductivelayer 41 b functions as the first drain side select gate line SGD1 andfunctions as the gate of the first drain side select transistor SDTr1.

The first source side conductive layer 41 a is formed on the upper layerof one columnar part 34 a constituting the memory semiconductor layer34, and the first drain side conductive layer 41 b is the same layer asthe first source side conductive layer 41 a and is formed on the upperlayer of the other columnar part 34 a constituting the memorysemiconductor layer 34. The first source side conductive layer 41 a andthe first drain side conductive layer 41 b is formed in a stripe shapethat has a prescribed pitch in the column direction and extends in therow direction. The first source side conductive layer 41 a and the firstdrain side conductive layer 41 b are composed of polysilicon (poly-Si).

The select transistor layer 40, as shown in FIG. 4, has the first sourceside hole 42 a and the first drain side hole 42 b. The first source sidehole 42 a is formed so that it penetrates through the first source sideconductive layer 41 a. The first drain side hole 42 b is formed so thatit penetrates through the first drain side conductive layer 41 b. Thefirst source side hole 42 a and the first drain side hole 42 b are,respectively, formed at the positions matching with the memory hole 32.

The select transistor layer 40, as shown in FIG. 5, has a first sourceside gate insulating layer 43 a, a first source side columnarsemiconductor layer 44 a, a first drain side gate insulating layer 43 b,and a first drain side columnar semiconductor layer 44 b. The firstsource side columnar semiconductor layer 44 a functions as the body ofthe first source side select transistor layer SSTr1. The first drainside columnar semiconductor layer 44 b functions as the body of thefirst drain side columnar semiconductor layer SDTr1.

The first source side gate insulating layer 43 a is formed at aprescribed thickness on the side surface of the first source side hole42 a. The first source side gate insulating layer 43 a has a blockinsulating layer 43 aa, a charge storage layer 43 ab, and a tunnelinsulating layer 43 ac. The charge storage layer 43 ab is a layer havinga function of accumulating electric charges.

The block insulating layer 43 aa, as shown in FIG. 5, is formed at aprescribed thickness on the side surface of the first source side hole43 a. The block insulating layer 43 aa is continuously formed integrallywith the block insulating layer 33 a. The charge storage layer 43 ab isformed at a prescribed thickness on the side surface of the blockinsulating layer 43 aa. The charge storage layer 43 ab is continuouslyformed integrally with the charge storage layer 33 b. The tunnelinsulating layer 43 ac is formed at a prescribed thickness on the sidesurface of the charge storage layer 43 ab. The tunnel insulating layer43 ac is continuously formed integrally with the tunnel insulating layer33 c. The block insulating layer 43 aa and the tunnel insulating layer43 ac are composed of a silicon dioxide (SiO₂). The charge storage layer43 ab is composed of a silicon nitride (SiN).

The first source side columnar semiconductor layer 44 a contacts withthe side surface of the first source side gate insulating layer 43 a andthe upper surface of one of a pair of columnar part 34 a and is formedin a columnar shape so that it extends in the direction perpendicular tothe substrate 10. The first source side columnar semiconductor layer 44a is formed so that the first source side hole 42 a is buried in it. Thefirst source side columnar semiconductor layer 44 a is continuouslyformed integrally with the columnar part 34 a. The first source sidecolumnar semiconductor layer 44 a is composed of polysilicon (poly-Si).

The first drain side gate insulating layer 43 b is formed at aprescribed thickness on the side surface of the first drain side hole 42b. The first drain side gate insulating layer 43 b has a blockinsulating layer 43 ba, a charge storage layer 43 bb, and a tunnelinsulating layer 43 bc. The charge storage layer 43 bb changes thethreshold voltage of the first drain side select transistor SDTr1 byaccumulating electric charges.

The block insulating layer 43 ba, as shown in FIG. 5, is formed at aprescribed thickness on the side surface of the first drain side hole 43b. The block insulating layer 43 ba is continuously formed integrallywith the block insulating layer 33 a. The charge storage layer 43 bb isformed at a prescribed thickness on the side surface of the blockinsulating layer 43 ba. The charge storage layer 43 bb is continuouslyformed integrally with the charge storage layer 43 bb. The tunnelinsulating layer 43 bc is formed at a prescribed thickness at the sidesurface of the charge storage layer 43 bb. The tunnel insulating layer43 bc is continuously formed integrally with the tunnel insulating layer33 c. The block insulating layer 43 ba and the tunnel insulating layer43 bc are composed of silicon dioxide (SiO₂). The charge storage layer43 bb is composed of silicon nitride (SiN).

The first drain side columnar semiconductor layer 44 b is in contactwith the side surface of the first drain side gate insulating layer 43 band the upper surface of the other of a pair of columnar parts 34 a andis formed in a columnar shape so that it extends in the directionperpendicular to the substrate 10. The first drain side columnarsemiconductor layer 44 b is formed so that the first drain side hole 42b is buried in it. The first drain side columnar semiconductor layer 44b is continuously formed integrally with the columnar part 34 a. Thefirst drain side columnar semiconductor layer 44 b is composed ofpolysilicon (poly-Si).

In addition, the select transistor layer 40, as shown in FIG. 4, has asecond source side conductive layer 45 a and a second drain sideconductive layer 45 b. The second source side conductive layer 45 afunctions as the second source side select gate line SGS2 and functionsas the gate of the second source side select transistor SSTr2. Thesecond drain side conductive layer 45 b functions as the second drainside select gate line SGD2 and functions as the gate of the second drainside select transistor SDTr2.

The second source side conductive layer 45 a is formed on the upperlayer of the first source side conductive layer 41 a. The second drainside conductive layer 45 b is the same layer as the second source sideconductive layer 45 a and is formed on the upper layer of the firstdrain side conductive layer 41 b. The second source side conductivelayer 45 a and the second drain side conductive layer 45 b are composedof polysilicon (poly-Si).

The select transistor layer 40, as shown in FIG. 4, has a second sourceside hole 46 a and a second drain side hole 46 b. The second source sidehole 46 a is formed so that it penetrates through the second source sideconductive layer 45 a. The second source side hole 46 a is formed at theposition matching with the first source side hole 42 a. The second drainside hole 46 b is formed so that it penetrates through the second drainside conductive layer 45 b. The second drain side hole 46 b is formed atthe position matching with the first drain side hole 42 b.

The select transistor layer 40, as shown in FIG. 5, has a second sourceside gate insulating layer 47 a, a second source side columnarsemiconductor layer 48 a, a second drain side gate insulating layer 47b, and a second drain side columnar semiconductor layer 48 b. The secondsource side columnar semiconductor layer 48 a functions as the body ofthe second source side select transistor SSTr2. The second drain sidecolumnar semiconductor layer 48 b functions as the body of the seconddrain side columnar semiconductor layer SDTr2.

The second source side gate insulating layer 47 a is formed at aprescribed thickness on the side surface of the second source side hole46 a. The second source side gate insulating layer 47 a has a blockinsulating layer 47 aa, a charge storage layer 47 ab, and a tunnelinsulating layer 47 ac. The charge storage layer 47 ab changes thethreshold voltage of the second source side select transistor SSTr2 byaccumulating electric charges.

The block insulating layer 47 aa, as shown in FIG. 5, is formed at aprescribed thickness on the side surface of the second source side hole46 a. The block insulating layer 47 aa is continuously formed integrallywith the block insulating layer 43 aa. The charge storage layer 47 ab isformed at a prescribed thickness on the side surface of the blockinsulating layer 47 aa. The charge storage layer 47 ab is continuouslyformed integrally with the charge storage layer 43 ab. The tunnelinsulating layer 47 ac is formed at a prescribed thickness on the sidesurface of the charge storage layer 47 ab. The tunnel insulating layer47 ac is continuously formed integrally with the tunnel insulating layer43 ac. The block insulating layer 47 aa and the tunnel insulating layer47 ac are composed of silicon dioxide (SiO₂). The charge storage layer47 ab is composed of silicon nitride (SiN).

The second source side columnar semiconductor layer 48 a is in contactwith the side surface of the second source side gate insulating layer 47a and the upper surface of the first source side columnar semiconductorlayer 44 a and is formed in a columnar shape so that it extends in thedirection perpendicular to the substrate 10. The second source sidecolumnar semiconductor layer 48 a is formed so that the second sourceside hole 46 a is buried in it. The second source side columnarsemiconductor layer 48 a is continuously formed integrally with thefirst source side columnar semiconductor layer 44 a. The second sourceside columnar semiconductor layer 48 a is composed of polysilicon(poly-Si).

The second drain side gate insulating layer 47 b is formed at aprescribed thickness on the side surface of second drain side hole 46 b.The second drain side gate insulating layer 47 b has a block insulatinglayer 47 ba, a charge storage layer 47 bb, and a tunnel insulating layer47 bc. The charge storage layer 47 bb changes the threshold voltage ofthe second drain side select transistor SDTr2 by accumulating electriccharges.

Therefore, the drain side select transistors SDTr1, SDTr2, SSTr1, andSSTr2 have the charge storage layers 43 ab, 43 bb, 47 ab, and 47 bbsimilar to those of the memory transistors MTr and can change thresholdvoltage by changing the amount of electric charges that are accumulatedin the charge storage layer.

Select transistors generally are not required to have a charge storagelayer. However, in this embodiment, these select transistors have thecharge storage layer, which helps simplify manufacturing process flowsand thus reduce manufacturing costs. In other words, when the selecttransistors are formed without a charge storage layer, the number ofprocess different steps is increased, rather than simply repeating thesame process steps multiple times. Accordingly, in this embodiment,after the conductive layers 31 a-31 d, the conductive layers 41 a, 41 b,45 a, and 45 b, and the interlayer dielectric sandwiched between theselayers, which is not shown in the drawing, are layered, a U-shaped holeis formed, and a silicon oxide film, a silicon nitride film (chargestorage layer), and a silicon oxide film are sequentially deposited onthe wall surface of the hole, though it is not shown in the drawing,thus obtaining a structure as shown in FIG. 5.

However, when the gate insulating layer of the select transistors has acharge storage layer, holes or electrons may be trapped in this chargestorage layer during a write operation or readout operation on thememory cells, causing a concern that the threshold voltage of the selecttransistors is accidentally changed during these steps. For this reason,in this embodiment, the control circuit AR2 is configured so that anadjusting operation (quasi-write operation) of the threshold voltage forthe select transistors can be implemented to alter/control the thresholdvoltage of the select transistors.

The block insulating layer 47 ba, as shown in FIG. 5, is formed at aprescribed thickness on the side surface of the second drain side hole46 b. The block insulating layer 47 ba is continuously formed integrallywith the block insulating layer 43 ba. The charge storage layer 47 bb isformed at a prescribed thickness on the side surface of the blockinsulating layer 47 ba. The charge storage layer 47 bb is continuouslyformed integrally with the charge storage layer 43 bb. The tunnelinsulating layer 47 bc is formed at a prescribed thickness on the sidesurface of the charge storage layer 47 bb. The tunnel insulating layer47 bc is continuously formed integrally with the tunnel insulating layer43 bc. The block insulating layer 47 ba and the tunnel insulating layer47 bc are composed of silicon dioxide (SiO₂). The charge storage layer47 bb is composed of silicon nitride (SiN).

The second drain side columnar semiconductor layer 48 b is in contactwith the side surface of the second drain side gate insulating layer 47b and the upper surface of the first drain side columnar semiconductorlayer 44 b and is formed in a columnar shape so that it extends in thedirection perpendicular to the substrate 10. The second drain sidecolumnar semiconductor layer 48 b is formed so that the second drainside hole 46 b is buried in it. The second drain side columnarsemiconductor layer 48 b is continuously formed integrally with thefirst drain side columnar semiconductor layer 44 b. The second drainside columnar semiconductor layer 48 b is composed of polysilicon(poly-Si), for example.

In other words, in the constitution of the select transistor layer 40,the first source side gate insulating layer 43 a is formed so that itencloses the first source side columnar semiconductor 44 a. The firstsource side conductive layer 41 a is formed so that it encloses thefirst source side columnar semiconductor layer 44 a via the first sourceside gate insulating layer 43 a. The first drain side gate insulatinglayer 43 b is formed so that it encloses the first drain side columnarsemiconductor layer 44 b. The first drain side conductive layer 41 b isformed so that it encloses the first drain side columnar semiconductorlayer 44 b via the first drain side gate insulating layer 43 b.

In addition, in the constitution of the select transistor layer 40, thesecond source side gate insulating layer 47 a is formed so that itencloses the second source side columnar semiconductor layer 48 a. Thesecond source side conductive layer 45 a is formed so that it enclosesthe second source side columnar semiconductor layer 48 a via the secondsource side gate insulating layer 47 a. The second drain side gateinsulating layer 47 b is formed so that it encloses the second drainside columnar semiconductor layer 48 b. The second drain side conductivelayer 45 b is formed so that it encloses the second drain side columnarsemiconductor layer 48 b via the second drain side gate insulating layer47 b.

The wiring layer 50, as shown in FIG. 4, is formed on the upper layer ofthe select transistor layer 40. The wiring layer 50 has a source linelayer 51 and a bit line layer 52. The source line layer 51 functions asthe source lines SL, and the bit line layer 52 functions as the bitlines BL.

The source line layer 51 is formed in a planar (plate) shape extendingin the row direction. The source line layer 51 is formed so that it isin contact with the upper surface of a pair of second source sidecolumnar semiconductor layers 48 a adjacent in the column direction. Thebit line layer 52 is in contact with the upper surface of the seconddrain side columnar semiconductor layer 48 b and is formed in stripeswith a prescribed pitch in the row direction and extending in the columndirection. The source line layer 51 and the bit line layer 52 arecomposed of metal such as tungsten (W).

Next, the shape of the second source side conductive layer 45 a and thesecond drain side conductive layer 45 b will be explained in detail withreference to FIG. 6. FIG. 6 is a top view showing the second source sideconductive layer 45 a and the second drain side conductive layer 45 b.

The second source side conductive layer 45 a and the second drain sideconductive layer 45 b, as shown in FIG. 6, are respectively formed in acomb shape from the vertical direction. The second source sideconductive layer 45 a includes several straight line parts 451 a, whichenclose several second source side columnar semiconductor layers 48 aparallel in the row direction, and a straight line part 452 a forconnecting the ends of several straight line parts 451 a. Similarly, thesecond drain side conductive layer 45 b includes several straight lineparts 451 b, which enclose several second drain side columnarsemiconductor layers 48 b parallel in the row direction, and a straightline part 452 b for connecting the ends of several straight line parts451 b. As shown in FIG. 6, four straight line parts 451 a and twostraight line parts 451 b are installed in an alternate fashion in thecolumn direction.

Next, a detailed constitution of the control circuit AR2 will beexplained with reference to FIG. 7. FIG. 7 is a circuit diagram showinga detailed constitution of the control circuit AR2. The control circuitAR2, as shown in FIG. 7, has an address decoder circuit 11, step-upcircuits 12 a, 12 a′, 12 b, and 12 c, word line drive circuits 13 a and13 b, a back gate line drive circuit 14, select gate line drive circuits15 a and 15 b, a source line drive circuit 16, a sense amplifier circuit17, a sequencer 18, and row decoder circuits 19 a and 19 b.

The address decoder circuit 11, as shown in FIG. 7, outputs a signal BADto the row decoder circuits 19 a and 19 b. The signal BAD is a signalfor designating the memory block MB (block address).

The step-up circuits 12 a, 12 a′, 12 b, and 12 c generate a step-upvoltage in which a reference voltage has been stepped up. In the step-upcircuits 12 a and 12 a′, as shown in FIG. 7, the stepped-up voltage istransferred to the word line drive circuits 13 a and 13 b. The step-upcircuits 12 a and 12 a′ respectively generate different voltages. Theformer generates a voltage that is supplied to selected word lines, andthe latter generates a voltage that is supplied to non-selected wordlines. For example, in a readout operation, the step-up circuit 12 agenerates a read voltage V_(CGRV), which is applied to selected wordlines, and the step-up circuit 12 a′ generates a read path voltage Vreadthat is applied to non-selected word lines. The read voltage V_(CGRV) isa voltage between the upper limit and the lower limit of severalthreshold voltage distributions, and the read pass voltage Vread is avoltage that is higher than the upper limit of the maximum thresholdvoltage distribution and can conduct the memory cells, regardless ofheld data of the memory cells. On the other hand, in a write operation,the step-up circuit 12 a generates a program voltage Vpgm (for example,20 V or higher) that is applied to selected word lines, and the step-upcircuit 12 a′ generates a write pass voltage Vpass (about 8-10 V) thatis applied to the non-selected word lines. The write voltage Vpgm is avoltage with a magnitude great enough for generating a tunnel current,which injects electrons into a floating gate of the memory cells, when 0V is applied to a channel. On the other hand, the write pass voltageVpass is a voltage with a magnitude insufficient for injecting electronsinto the floating gate, even when 0 V is applied to the channel.

In addition, the step-up circuit 12 b outputs a stepped-up voltage tothe source line drive circuit 16. The step-up circuit 12 c outputs astepped-up signal RDEC to the row decoder circuits 19 a and 19 b.

The word line drive circuit 13 a, as shown in FIG. 7, outputs signalsVCG1-4. The word line drive circuit 13 b outputs signals VCG5-8. Thesignals VCG1-8 are used in driving the word lines WL1-8 of a selectmemory block MB<i>.

The back gate line drive circuit 14, as shown in FIG. 7, outputs asignal VBG. The signal VBG is used in driving the back gate line BG ofthe select memory block MB<i>.

The select gate line drive circuit 15 a, as shown in FIG. 7, outputssignal VSGSb, signal VSGDa, signal VSGD2, and signal VSGOFF. The selectgate line drive circuit 15 b outputs signal VSGSa, signal VSGDb, signalVSGS2, and signal VSGOFF. The signal VSGSa and the signal VSGSb arerespectively used in driving the first source side select gate line SGS1of the first column and the second column of the select memory blockMB<i>. Signal VSGDa and signal VSGDb are respectively used in drivingthe first drain side select gate line SGD1 of the first column and thesecond column of the select memory block MB<i>. The signal VSGS2 is usedin driving the second source side select gate line SGS2 of the selectmemory block MB<i>. The signal VSGD2 is used in driving the second drainside select gate line SGD2 of the select memory block MB<i>. The signalVSGOFF is used in driving the first source side select gate line SGS1and the first drain side select gate line SGD1 of the nonselect memoryblock MB<i>.

Here, the signal VSGSb, signal VSGDa, and signal VSGOFF are input intovarious kinds of wirings via the row decoder circuit 19 a from theselect gate line drive circuit 15 a. On the other hand, the signal VSGD2is directly input as a signal VSGD2<i> into the gate of the second drainside select transistor SDTr2 from the select gate line drive circuit 15a. In addition, the signal VSGOFF, signal VSGDb, and signal VSGSa areinput into various kinds of wirings via the row decoder circuit 19 bfrom the select gate line drive circuit 15 b. On the other hand, thesignal VSGS2 is directly input as a signal VSGS2<i> into the gate of thesecond source side select transistor SSTr2 from the select gate linedrive circuit 15 b. Moreover, the signals VSGS2 and VSGD2 are suppliedas the common signals over several memory blocks MB.

The source line drive circuit 16, as shown in FIG. 7, outputs a signalVSL. The signal VSL is used in driving the source lines SL.

The sense amplifier circuit 17, as shown in FIG. 7, charges a prescribedbit line BL up to a prescribed voltage by outputting a signal VBL<i> andthen decides held data of the memory transistor MTr in the memory stringMS based on the change of the voltage of the bit lines BL. In addition,the sense amplifier circuit 17 outputs the signal VBL<i> correspondingto the write data to the prescribed bit line BL.

The sequencer 18, as shown in FIG. 7, supplies a control signal to thecircuits 11-17 and controls these circuits.

One each of the row decoder circuits 19 a and 19 a, as shown in FIG. 5,is respectively installed for one memory block MB. The row decodercircuit 19 a inputs signal VCG1<i>-VCG4<i> into the gates of the memorytransistors MTr1-MTr4 based on the signal BAD and the signals VCG1-VCG4.In addition, the row decoder circuit 19 a selectively inputs a signalVSGSb<i> into the gate of the first source side select transistor SSTr1of the second column of the memory units MU based on the signals BAD,VSGSb, and SGOFF. Moreover, the row decoder circuit 19 a selectivelyinputs a signal VSGDa<i> into the gate of the first drain side selecttransistor SDTr1 of the first column of the memory units MU based on thesignals BAD, VSGDa, and SGOFF.

The row decoder circuit 19 a has a NAND circuit 19 aa, a NOT circuit 19ab, a voltage converting circuit 19 ac, first transfer transistorsTra1-Tra6, and second transfer transistors Trb1 and Trb2. The voltageconverting circuit 19 ac generates a signal VSELa<i> based on thesignals BAD and RDEC received via the NAND circuit 19 aa and the NOTcircuit 19 ab and outputs the signal to the gates of the first transfertransistors Tra1-Tra6. In addition, the voltage converting circuit 19 acgenerates a signal VbSELa<i> based on the signals BAD and RDEC andoutputs the signal to the gates of the second transfer transistors Trb1and Trb2.

The first transfer transistors Tra1-Tra4 are connected between the wordline drive circuit 13 a and each word line WL1-WL4. The first transfertransistors Tra1-Tra4 output the signals VCG1<i>-VCG4<i> to the wordlines WL1-WL4 based on the signals VCG1-VCG4 and VSELa<i>. The firsttransfer transistor Tra5 is connected between the select gate line drivecircuit 15 a and the first source side select gate line SGS1 of thesecond column of the memory units MU. The first transfer transistor Tra5outputs the signal VSGSb<i> to the first source side select gate lineSGS1 of the second column of the memory units MU based on the signalsVSGSb and VSELa<i>. The first transfer transistor Tra6 is connectedbetween the select gate line drive circuit 15 a and the first drain sideselect gate line SGD1 of the first column of the memory units MU. Thefirst transfer transistor Tra6 outputs the signal VSGDa<i> to the firstdrain side select gate line SGD1 of the first column of the memory unitMU based on the signals VSGDa and VSELa<i>.

The second transfer transistor Trb1 is connected between the select gateline drive circuit 15 a and the second column of the first source sideselect gate line SGS1. The second transfer transistor Trb1 outputs thesignal VSGSb<i> to the first source side select gate line SGS1 of thesecond column of the memory units MU based on the signals VSGOFF andVbSELa<i>. The second transfer transistor Trb2 is connected between theselect gate line drive circuit 15 a and the drain side select gate lineSGD of the first column of the memory units MU. The second transfertransistor Trb2 outputs the signal VSGDa<i> to the first drain sideselect gate line SGD1 of the first column of the memory unit MU based onthe signals VSGOFF and VbSELa<i>.

The row decoder circuit 19 b inputs signals VCG5<i>-VCG8<i> into thegates of the memory transistors MTr5-MTr8 based on the signals BAD andVCG5-VCG8. In addition, the row decoder circuit 19 b selectively inputsa signal VSGSa<i> into the gate of the first source side selective SSTr1of the first column of the memory unit MU based on the signals BAD,VSGSa, and SGOFF. In addition, the row decoder circuit 19 b selectivelyinputs a signal VSGDb<i> into the gate of the first drain side selecttransistor SDTr1 of the second column of the memory units MU based onthe signals BAD, VSGDb, and SGOFF.

The row decoder circuit 19 b has a NAND circuit 19 ba, a NOT circuit 19bb, a voltage converting circuit 19 bc, first transfer transistorsTrc1-Trc7, and second transfer transistors Trd1 and Trd2. The voltageconverting circuit 19 bc generates a signal VSELb<i> based on thesignals BAD and RDEC received via the NAND circuit 19 ba and the NOTcircuit 19 bb, and outputs the signal to the gates of the first transfertransistors Trc1-Trc7. Moreover, the voltage converting circuit 19 bcgenerates a signal VbSELb<i> based on the signals BAD and RDEC, andoutputs the signal to the gates of the second transfer transistors Trd1and Trd2.

The first transfer transistors Trc1-Trc4 are connected between the wordline drive circuit 13 b and each word line WL5-WL8. The first transfertransistors Trc1-Trc4 output the signals VCG5<i>-VCG8<i> to the wordlines WL5-WL8 based on the signals VCG5-VCG8 and VSELb<i>. The firsttransfer transistor Trc5 is connected between the back gate line drivecircuit 14 and the back gate line BG. The first transfer transistor Trc5outputs a signal VBG<i> to the back gate line BG based on the signalsVBG and VSELb<i>. The first transfer transistor Trc6 is connectedbetween the select gate line drive circuit 15 b and the first sourceside select gate line SGS1 of the first column of the memory units MU.The first transfer transistor Trc6 outputs a signal VSGSa<i> to thefirst source side select gate line SGS1 of the first column of thememory units MU based on the signals VSGSa and VSELb<i>. The firsttransfer transistor Trc7 is connected between the select gate line drivecircuit 15 b and the first drain side select gate line SGD1 of thesecond column of the memory units MU. The first transfer transistor Trc7outputs a signal VSGDb<i> to the first drain side select gate line SGD1of the second column of the memory units MU based on the signals VSGDband VSELb<i>.

The second transfer transistor Trd1 is connected between the select gateline drive circuit 15 b and the first source side select gate line SGS1of the first column of the memory units MU. The second transfertransistor Trd6 outputs the signal VSGSa<i> to the first source sideselect gate line SGS1 of the first column of the memory units MU basedon the signals VSGOFF and VbSELb<i>. The second transfer transistor Trd2is connected between the select gate line drive circuit 15 b and thefirst drain side select gate line SGD1 of the second column of thememory units MU. The second transfer transistor Trd2 outputs the signalVSGDb<i> to the first drain side select gate line SGD1 of the secondcolumn of the memory units MU based on the signals VSGOFF and VbSELb<i>.

Next, a detailed circuit configuration of the step-up circuit 12 a ofthis embodiment will be explained with reference to FIG. 8. Here, theother step-up circuits 12 a′, 12 b, and 12 c have constitutions shown inFIG. 9; however, in the following examples, it is assumed that only thestep-up circuit 12 a has the constitution of FIG. 8.

As shown in FIG. 8, the step-up circuit 12 a is provided with anamplifier part (module) 100 and a current replica circuit 200. Theamplifier part 100 generates a step-up voltage, which is supplied to theword lines WL, based on a power supply voltage. The current replicacircuit 200 generates a replica current Ireplica proportional to acurrent flowing in the amplifier part (module) 100 and controls theamplifier part 100 based on the replica current Ireplica.

The amplifier part 100 is provided with a charging circuit 110 and adischarging circuit 120. The charging circuit 110 includes a PMOStransistor M1 and a differential amplifier 111. The PMOS transistor M1is connected so that a circuit is formed between a power supply voltageterminal and an output terminal AMPOUT (node N1), and an output terminalof the differential amplifying circuit 111 is connected to its gate. Thedifferential amplifying circuit 111 differentially amplifies the voltageof the node N1 and a reference voltage VREF, and outputs a differentialamplified signal. In this way, the voltage of the node N1 is controlled.

The discharging circuit 120 includes NMOS transistors M2-M5. The NMOStransistors M2 and M3 are connected in series between the node N1 and aground terminal, and the NMOS transistors M4 and M5 are connected inseries between the node N1 and the ground terminal.

The NMOS transistor M4 conducts when a bias VIREFN1 is applied to itsgate. The NMOS transistor M5 conducts when an enable signal EN isapplied. The enable signal EN is switched to “H” at the same time of thestart of the operation of the step-up circuit 12 a.

In addition, the NMOS transistor M2 conducts when a bias VIREFN2 isapplied to its gate. The NMOS transistor M3 conducts when the enablesignal ENB, which is output from the current replica circuit 200, isapplied. The enable signal ENB, as will be described later, is turned to“H” when the replica current Ireplica is lowered to a prescribed valueor greater. Therefore, a current path, which is formed by the NMOStransistors M2 and M3, is usually in a nonconductive state (cut-offstate) and is set to a conductive state only when the replica currentIreplica is lowered to a prescribed value or more.

Moreover, the current replica circuit 200 includes a PMOS transistor M6and a current level detector 210. The PMOS transistor M6 is connected tothe PMOS transistor M1 and a current mirror. In other words, the sourceof the PMOS transistor M6 is connected to the power supply voltageterminal, and its drain is connected to the current level detector 210.Furthermore, its gate is connected to the gate of the PMOS transistorM1.

Next, the operation of the step-up circuit 12 a will be explained withreference to a timing charge of FIG. 9. At time to, if the enable signalEN is switched to “H”, the operation of the step-up circuit 12 a isstarted. The output voltage of the output terminal AMPOUT is raised upto the voltage V_(CGRV) that is determined by the reference voltageVREF. The current path including the NMOS transistors M4 and M5 sends acurrent I2, and the differential amplifier 111 controls a current I1. Atthat time, since the enable signal ENB is at “L”, no current flows tothe NMOS transistors M2 and M3. With the balance of the currents I1 andI2, the voltage of the output terminal AMPOUT is controlled to thevoltage V_(CGRV).

At time t1, if the voltage of non-selected word lines WL (adjacent WL)adjacent to selected word lines starts to be raised by the operation ofthe step-up circuit 12 a′, the voltage of the selected word lines WL(the voltage of the output terminal AMPOUT) is raised by capacitivecoupling (coupling noise is generated).

If the voltage of the output voltage AMPOUT is raised, the current I1 islowered by the action of the differential amplifier 111. Along with it,the value of the replica current Ireplica is also lowered. The replicacurrent Ireplica is compared with the reference current Iref in thecurrent level detector 210. Next, if the current level detection circuit210 decides that the replica current Ireplica reaches the referencecurrent Iref or lower, the current level detector 210 switches theenable signal ENB to “H.” Therefore, the current path of the NMOStransistors M2 and M3 is set to a conductive state (the current of thecurrent path is changed from zero to Itail2 (>0), pulling down thevoltage of the node N1. Thereby, the voltage of the selected word linesWL (output terminal AMPOUT) raised by the capacitive coupling is rapidlyconverged to the original, intended voltage V_(CGRV). If the voltage ofthe output terminal AMPOUT returns to the voltage V_(CGRV), the currentI1 also returns to the original value, so that the enable signal ENBalso falls to “L”. In this manner, the voltage of the selected word lineWL can be rapidly returned to a desired value, even if it is raised bycapacitive coupling.

FIG. 10 shows a detailed constitutional example of the current leveldetector 210 of the current replica current 200. The current leveldetector 210 includes differential amplifying circuit 211, resistor R1,and NMOS transistor M7.

The resistor R1 and the NMOS transistor M7 are connected in seriesbetween the drain (node N2) of the PMOS transistor and the groundterminal. The NMOS transistor M7 is operated when the enable signal EN2is applied. The differential amplifier 211 differentially amplifies thevoltage of the reference voltage VREF1 and the voltage of the node N2and outputs the enable signal ENB.

(Effects of First Embodiment)

According to the first embodiment, the voltage in wirings raised bycapacitive coupling can be rapidly returned to a desired voltage, thusbeing able to improve the performance of the device.

(Second Embodiment)

Next, the semiconductor memory device of the second embodiment will beexplained with reference to FIG. 11. Since the semiconductor memorydevice is similar to that of the first embodiment, detailed explanationof common aspects will be omitted in the following. However, the secondembodiment differs from first embodiment in the configuration of thecurrent replica circuit 200 of the step-up circuit 12 a.

The constitution of the current replica circuit 200 of the secondembodiment will be explained with reference to FIG. 11. In the currentlevel detector 210 of the current replica circuit 200, NMOS transistorsM7 and M8 are connected in series between the node N2 and the groundterminal. A bias VIREFN3 is applied to the gate of the NMOS transistorM7, and the enable EN2 is applied to the gate of the NMOS transistor M8.Therefore, the NMOS transistors M7 and M8 function as constant currentcircuits for sending a constant current Iconst to their current path.

In addition, capacitors C1 and C2 are connected in series between thenode N2 and the ground terminal. An input terminal of an inverter INV1is connected to a connecting node N3 between the capacitors C1 and C2.An output signal of the inverter INV1 is input into a logic circuit 212,and an output signal of the logic circuit 212 is output as the enablesignal ENB to the gate of the NMOS transistor M3 via a level shifter213. Here, the NMOS transistor M9 conducts by an inverted signal/EN ofthe enable signal EN and precharges the potential of the node n3.

The current I1 in this embodiment is lowered by the same principle asthat of the first embodiment, and the replica current Ireplica islowered along with it, so that electric charges of the capacitors C1 andC2 flow out via the NMOS transistors M7 and M8, thereby lowering thepotential of the node N3. If the potential of the node N3 reaches aprescribed value or smaller, the enable signal ENB rises to “H”. Inother words, the same effects as those of the first embodiment can beachieved.

FIG. 12 shows the step-up circuit 12 a of a modified example of thesecond embodiment. In FIG. 12, the same symbols will also be given tothe same constituent elements as those of FIG. 11, and the explanationfor common features will be omitted in the following. In this modifiedexample, the discharging circuit 120 is provided with four current pathsincluding NMOS transistors M4, M5, M21, M22, M23, M31, M32, and M33. TheNMOS transistors M21 and M31 are connected in series between the node N1and the ground terminal, the NMOS transistors M22 and M32 are connectedin series between the node N1 and the ground terminal, and the NMOStransistors M23 and M33 are connected in series between the node N1 andthe ground terminal. The NMOS transistors M31-M33 are respectivelyconducted when the enable signals ENB1-ENB3, which are transmitted fromlevel shifters 213 a-213 c, are rendered.

The logic circuit 212 has a function of determining all of the enablesignals ENB1-3 to “H”, or selectively determining only part of theenable signals to “H”, when a signal PLANE for specifying a plane, inwhich the memory cell array AR1 is formed, and an enable signal ENB0 aretransmitted and the current Ireplica is lower than the reference currentIref based on the signal PLANE. According to the configuration of FIG.12, the discharge performance of the discharging circuit 120 can bechanged for each plane. Therefore, variations in the characteristicsbetween the planes can be absorbed.

OTHER EXAMPLES

For example, in the aforementioned embodiments, the three-dimensionalNAND-type flash memory has been explained as an example; however, theembodiments can also be applied to other memories, for example, a planarNAND-type flash memory. In addition, in the aforementioned embodiments,an example in which the current path of the discharging circuit 120 iscut off or conducted by the enable signal ENB has been explained.However, without being limited to this, the embodiment may be applied toother cases as long as the amount of current flowing in the current pathcan be changed.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the embodiments described herein may beembodied in a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the embodiments describedherein may be made without departing from the spirit of the inventions.The accompanying claims and their equivalents are intended to cover suchforms or modifications as would fall within the scope and spirit of theinventions.

The structure of the memory cell array is not limited to as describedabove. A memory cell array formation may be as disclosed in U.S. patentapplication Ser. No. 12/532,030, the entire contents of which areincorporated by reference herein.

What is claimed is:
 1. A semiconductor memory device, comprising: amemory cell array in which a plurality of memory cells are arranged; afirst wiring connected to a memory cell; a discharging circuitconfigured to discharge a voltage of the first wiring according to afirst current; a charging circuit configured to charge the voltage ofthe first wiring according to a second current; a control circuitconfigured to detect the voltage of the first wiring and control amagnitude of the second current based on the detected voltage; and acurrent detection unit configured to generate a third current that isproportional to the second current and a detection result, thedischarging circuit is configured to control a magnitude of the firstcurrent in accordance with the detection result.
 2. The semiconductormemory device according to claim 1, wherein the charging circuitcomprises a first transistor; and the current detection unit comprises asecond transistor having a current mirror connection with the firsttransistor.
 3. The semiconductor memory device according to claim 2,wherein the discharging circuit has a plurality of current paths; and atleast one current path of the plurality is configured to allow themagnitude of the first current to be changed in accordance with thedetection result.
 4. The semiconductor memory device according to claim2, wherein the current detection unit comprises: a capacitor connectedbetween a first end portion of the second transistor and a groundterminal; and a constant current circuit connected between the first endportion of the second transistor and the ground terminal.
 5. Thesemiconductor memory device according to claim 1, wherein thedischarging circuit comprises a plurality of current paths, and at leastone current path of the plurality is configured to allow the magnitudeof the first current to be changed in accordance with the detectionresult.